Geochemistry, Petrology, and Thermal Properties of Lava from Fissure 17 in the 2018 Kilauea Eruption
In 2018, a complex lava erupted from Fissure 17 during the Lower East Rift Zone (LERZ) eruption of Kilauea, Hawaii. Fissure 17 was active from May 13-25 of 2018, and was more explosive than other vents, which produced fluid basalt. The Fissure 17 lavas plotted on a linear mixing trend between basalt and icelandite (iron-rich aluminum-poor andesite). Mixing of two different lavas is visible in hand specimen, with rigid crystalline inclusions in a microcrystalline groundmass that sheared around them. Our preliminary hypothesis was fresh basaltic lava encountering a cool crystal-rich evolved mush pocket from a previous magmatic episode; possibly a 1955 lava, creating icelandite inclusions in basalt. However, geochemical analyses indicate that the inclusions are basaltic and surrounded by andesitic lava. Fissure 17 lavas are consistent with the MELTS mixing model by Gansecki et al. (2019). Based on the petrography and thermal properties, a second source of basaltic material involved in mixing was added. This matches geochemically with the original (undifferentiated) 1955 lava. Calorimetry was used to determine the glass content of the icelandite groundmass (~50%) and the basaltic inclusions (undetectable). There is a possibility that inclusions could be xenoliths of 1955 lava. The conclusion is that, although in the field this lava is visibly hybridized, with numerous inclusions, these inclusions are not the major source of the geochemical variation in the suite. Instead of basalt containing andesitic inclusions, we have andesite / basaltic andesite containing basaltic xenoliths. The majority of the mixing occurred between two crystal-poor magmas, one already the product of mixing between two basaltic end-members and is now invisible. The broader implication is that visible evidence of mixing may tell a different story to the geochemical evidence in a single rift zone eruption.